1. Field of the Invention
The present invention relates to a thin film transistor substrate and a liquid crystal display panel for use in a liquid crystal display apparatus.
2. Description of the Related Art
In recent years, liquid crystal display apparatus have been used in television receivers, monitors and the like. In particular, in a liquid crystal display apparatus equipped with a liquid crystal display panel using a thin film transistor substrate having a matrix arrangement of thin film transistors (TFTs) on an insulating substrate (hereinafter referred to as “TFT liquid crystal display panel”), it is desired to improve image display quality with increased spatial and image depth resolution.
A TFT liquid crystal display panel generally includes a thin film transistor substrate, a counter substrate facing the thin film transistor substrate, and a liquid crystal layer sandwiched therebetween. The TFT liquid crystal display panel controls the gray scale of the image to be displayed by applying a voltage across transparent electrodes formed on the substrates to tilt liquid crystal molecules and change the light transmittance. Recent growth in demand of liquid crystal display apparatus has diversified requirements therefor. Among others, there is a strong demand for improvements in viewing angle characteristics and display quality, and vertically aligned (VA) liquid crystal display apparatus emerges as a potential candidate to meet this demand.
A VA liquid crystal display apparatus is characterized by vertically aligned films provided on the opposing surfaces of the substrates and a liquid crystal layer with negative dielectric anisotropy therebetween. The VA liquid crystal display apparatus also has, for example, linear domain restrictors (projections or slits) on the substrates to provide multi-domain alignment. This allows the VA liquid crystal display apparatus to achieve excellent viewing angle characteristics and display quality.
Although in TFT liquid crystal display panels used in the VA type and other types of liquid crystal display apparatus, each pixel typically has one pixel electrode, a structure in which the pixel electrode is divided into a plurality of sub-pixel electrodes in one pixel is also known in the art. For example, Japanese Patent No. 3,076,938 discloses a configuration in which in a pixel region defined by a gate bus line and drain bus line, electrical potentials supplied to a plurality of sub-pixel electrodes through one TFT are changed by capacitive coupling. JP-A-63-262621, JP-A-3-024524, and JP-A-9-179141 disclose thin film transistor substrates and liquid crystal display apparatuses having sub-pixel electrodes divided by a gate bus line or drain bus line.
Multi-domain vertical alignment liquid crystal display apparatus, however, has a problem that the screen looks whitish when viewed from a diagonal direction, which is so-called “white-blurring”. To address this problem, a method has been proposed in which the pixel electrode in one pixel is divided into a plurality of sub-pixel electrodes and distributed in a mixed manner such that some of the sub-pixel electrodes directly receive a gray level voltage through a pixel TFT and the other sub-electrodes have a predetermined voltage lower than the gray-level voltage due to capacitive coupling, which is a so-called capacitive half-tone driving method.
Japanese Patent No. 3098345 and JP-A-2002-287712 are exemplified as another related art documents.
However, the pixel structure used in the capacitive half-tone driving method has the following disadvantages. First, since a capacitive electrode for capacitively coupling the sub-pixel electrodes is formed of the same film in the pixel region as the opaque metal film forming the pixel TFT, the effective area of the pixel and hence the transmittance decrease due to the presence of the capacitive electrode. Second, to avoid such a problem, forming the capacitive electrode with a transparent electrode material increases the number of manufacturing steps. Third, insulation between the adjacent sub-pixel electrodes requires a slit region of a predetermined width, resulting in decreased effective pixel area by the amount of the slit width and hence decreased transmittance. Fourth, to avoid such a problem, decreasing the slit width may cause short circuit between the adjacent sub-pixel electrodes due to pattern defects during patterning of the sub-pixel electrodes. Fifth, the adjacent sub-pixel electrodes having different electrical potentials result in a complex pixel structure. Sixth, the complex pixel structure limits the freedom in pixel design.